A differential-type sensor, such as a differential transformer, can possess high sensitivity using contrary outputs of elements. For example, a sensor using strain gauges can generate improved output when those gauges are attached at positions where strains are generated in a reverse direction. Such sensor output is often used after amplification by operational amplifier.
Recently, various and extensive sensor information must be processed in computer control of vehicular fuel consumption and ecologically to reduce electric power consumption. A sensor circuit that is small and uses a reduced amount of electric power for processing signals and realizing high sensitivity is in demand. (Reference: "Sensor Electronics", Kiyoshi TAKAHASHI, et al., Shokodo, 1984).
FIG. 24 shows a capacitive sensor circuit of a conventional sensor as disclosed in Japanese Patent Laid-Open Publication No. 56-166411, in which an integrating circuit is formed by resistor R1 and capacitive sensor C1. A pulse is generated, which corresponds to the change of C1 is counted by counter CNT, and the output of CNT is converted into a voltage value by the integrating circuit.
FIG. 25 shows another capacitive sensor circuit of a conventional sensor as disclosed in Japanese Patent Publication No. 2-22338, in which a predetermined pulse is input to the first integrating circuit including resistor R2 and capacitive sensor C2, and to the second integrating circuit including resistor R1 and capacitor C1, the difference between the two is output as a voltage signal, and the change of C2 is detected at the output.
The above sensor circuits consume a lot of electric power because the circuits constantly and invariably generate pulses. Also, the circuits need complicated signal processing circuits like counter circuits, differential amplifying circuits, and pulse generating circuits. The resulting large circuit size makes it difficult to combine these circuits with sensors.